Self-suspending modified proppant system for carbon dioxide based fracturing fluids

ABSTRACT

Self-suspending proppants including proppant particles coated with a CO 2 -philic coating are provided. The CO 2 -philic coating may be lightly crosslinked and may have a physical structure that constrains CO 2  molecules. Methods of making self-suspending proppants may include coating a proppant particle with a polymerizable precursor material of a CO 2 -philic material and polymerizing the polymerizable precursor material to form a self-suspending proppant are also provided. Additionally, hydraulic fracturing fluids that may include a CO 2 -based fluid and the self-suspending proppants and methods of treating subterranean formations by contacting a subterranean formation with hydraulic fracturing fluid and propagating at least one subterranean fracture are provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 15/702,310 filed Sep. 12, 2017, which claims the benefit of priority to U.S. Provisional Application Ser. No. 62/458,132, filed Feb. 13, 2017, which is incorporated by reference in its entirety.

BACKGROUND Field

Embodiments of the present disclosure generally relate to self-suspending proppant systems. Specifically, embodiments of the present disclosure relate to self-suspending proppants and methods of making the same, and carbon dioxide-based hydraulic fracturing fluids and methods of using the same to treat subterranean formations.

Technical Background

Carbon dioxide (CO₂) may be used in hydraulic fracturing fluids to provide non-aqueous alternatives to conventional water-based fluids, as some subterranean formations may be sensitive to water. Moreover, ‘water-less’ fluids are more environmentally sound, minimizing depletion of natural-source freshwater often consumed in fracturing applications. However, conventional CO₂-based fluids are not sufficiently viscous to suspend propping agents, such as “proppants,” that are added to fracturing fluids to hold open subterranean fractures during and following fracturing treatment. Because of this reduced ability to suspend proppants, CO₂-based fluids have not been widely considered for use in fracturing fluids.

Accordingly, a need exists for non-aqueous hydraulic fracturing fluids that adequately support and suspend proppant particles. Historically, it has been very difficult to thicken CO₂-based fluids using additives, as CO₂ is not a good solvent for high molecular-weight polymers. Though water-based fluids may be easily viscosified owing to the many available thickening agents that readily dissolve in water, CO₂-based fluids are not easily viscosified, owing to a lack of available materials that are CO₂-soluble.

SUMMARY

Some embodiments of the present disclosure are directed to self-suspending proppants that include proppant particles coated with a CO₂-philic coating. The CO₂-philic coating is lightly crosslinked and has a physical structure that constrains or is solvated by CO₂ molecules.

Further embodiments of the present disclosure include hydraulic fracturing fluids composed of a CO₂-based fluid and a self-suspending proppant. The self-suspending proppant is a proppant particle coated with a CO₂-philic coating. The CO₂-philic coating is lightly crosslinked and has a physical structure that constrains CO₂ molecules.

Still further embodiments of the present disclosure include methods of producing self-suspending proppants. The methods of producing self-suspending proppants include coating a proppant particle with a polymerizable precursor material that includes a CO₂-philic component to form a coated proppant, and polymerizing the polymerizable precursor material to form the self-suspending proppant. The self-suspending proppant is a proppant particle coated with a CO₂-philic coating. The CO₂-philic coating is lightly crosslinked and has a physical structure that constrains CO₂ molecules.

Still further embodiments of the present disclosure include methods for treating subterranean formations. The methods for treating subterranean formations include contacting a subterranean formation with a hydraulic fracturing fluid comprising self-suspending proppant particles, and propagating at least one subterranean fracture in the subterranean formation. The hydraulic fracturing fluids composed of a CO₂-based fluid and a self-suspending proppant. The self-suspending proppant is a proppant particle coated with a CO₂-philic coating. The CO₂-philic coating is lightly crosslinked and has a physical structure that constrains CO₂ molecules.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings.

FIG. 1 is a schematic view of a proppant, a coated proppant, and a self-suspending proppant, according to embodiments shown and described.

FIG. 2A is a schematic view of a hydraulic fracturing fluid comprising uncoated proppants.

FIG. 2B is a schematic view of a hydraulic fracturing fluid according to embodiments shown and described.

FIG. 3 is a schematic view of a subterranean fracture being contacted with a hydraulic fracturing fluid according to embodiments shown and described.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to self-suspending proppants that include proppant particles coated with a CO₂-philic coating. The CO₂-philic coating may be lightly crosslinked and may have a physical structure that constrains CO₂ molecules. The proppant particle coated with the CO₂-philic coating may be suspendable in CO₂-based fluids such as hydraulic fracturing fluids, for example. The hydraulic fracturing fluids may be used in methods for treating subterranean formations. Embodiments also include methods of making the self-suspending proppants and of making hydraulic fracturing fluids comprising self-suspending proppants. Further embodiments include methods of using hydraulic fracturing fluids to treat a subterranean formation.

Subterranean formations such as rock, coal, or shale are treated by pumping a hydraulic fracturing fluid containing proppants into an opening in the formation to aid propagation of a fracture. The pressure of the injecting fluid causes the formation to fracture, and while the fluid is allowed to flow back to the surface, the proppants remain in the fracture and prevent the formation from closing or collapsing. Conventionally, CO₂-based fracturing fluids have such a low viscosity that proppant particles added to the fluid immediately sink and are not pumped into the fracture until the last of the fluid has been used. As a result, the fracture may close prematurely.

Embodiments of the present disclosure address these difficulties by providing self-suspending proppant particles for use in CO₂-based fluids and simplified, economical methods for treating subterranean formations with hydraulic fracturing fluids containing the self-suspending proppant particles suspended in a CO₂-based fluid. The self-suspending proppants are proppant particles coated with a CO₂-philic material that is lightly crosslinked and has a physical structure that can constrain CO₂ molecules. As used throughout the disclosure, “CO₂-philic” refers to a molecule, ion, polymer, or composition having an affinity or attraction to CO₂. The term “CO₂-based fluid” refers to a fluid that includes CO₂.

Specific embodiments will now be described with references to the figures. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. As used throughout this disclosure, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” component includes aspects having two or more such components, unless the context clearly indicates otherwise.

FIG. 1 schematically portrays three states of a proppant particle 110. On the left, an uncoated proppant particle 110 is depicted in a first, uncoated state. Then, in the middle, a coated proppant particle 142 is depicted in which the proppant particle 110 is in a second, coated state. Finally, on the right is a swollen self-suspending proppant 140, in which the proppant particle 110 is in a third, swollen state. In the first state, the proppant particle 110 is not coated and, when placed in a CO₂-based fluid, it would likely sink to the bottom of the fluid on account of the low viscosity of CO₂-based fluids. In the second state, the proppant particle 110 has undergone a coating step 310 to be coated with a CO₂-philic coating 120, forming a coated proppant particle 142. In the third state, the coated proppant particle 142 has undergone a contacting step 320 to be contacted with CO₂ molecules, forming a swollen self-suspending proppant 140 that is coated with a now swollen CO₂-philic coating 121. Upon contact with CO₂ molecules during the contacting step 320, the CO₂-philic coating 120 has volumetrically expanded to a swollen state, transforming the coated proppant particle 142 into a swollen self-suspending proppant 140.

As illustrated by FIG. 1, in some embodiments, the coated proppant particle 142 and the swollen self-suspending proppant 140 both include at least one proppant particle 110 coated with a CO₂-philic coating 120, 121. The proppant particle 110 may be chosen from any type of proppant suitable for use in hydraulic fracturing applications. As previously described, proppants are propping agent particles used in hydraulic fracturing fluids to maintain and hold open subterranean fractures during or following subterranean treatment. In some embodiments, the proppant particle 110 may comprise particles of materials such as oxides, silicates, sand, ceramic, resin, plastic, mineral, glass, or combinations thereof. For instance, the proppant particle 110 may comprise graded sand, treated sand, resin-coated sand, ceramic proppants, plastic proppants, low-density proppants, or other resin-coated materials. The proppant particle 110 may comprise particles of bauxite or of sintered bauxite. The proppant particle 110 may comprise glass particles or glass beads. Embodiments of the present disclosure may utilize at least one proppant particle 110 and in embodiments in which more than one proppant particle 110 is used, the proppant particles 110 may contain a mixture of two or more different materials or three or more different materials.

The material of the proppant particle 110 may be chosen based on the particular application and characteristics desired in a swollen self-suspending proppant 140. For instance, ceramic proppant materials may be suitable in embodiments desiring high strength, uniform size and shape, high thermal resistance and high conductivity. Fully or partially cured resin-coated sand may be chosen in embodiments to provide particles of irregular size and shape with medium crush resistance strength and medium conductivity. Sands may be chosen in embodiments desiring naturally occurring and cost effective proppants or low particle strength and low conductivity.

The proppant particle 110 may have any size and shape. In some embodiments, the one or more proppant particles 110 may have sizes from 8 mesh to 140 mesh (diameters from 106 micrometers (μm) to 2.36 millimeters (mm)). In some embodiments, the proppant particles 110 may have sizes from 16 mesh to 30 mesh (diam. 600 μm to 1180 μm), 20 mesh to 40 mesh (diam. 420 μm to 840 μm), 30 mesh to 50 mesh (diam. 300 μm to 600 μm), 40 mesh to 70 mesh (diam. 212 μm to 420 μm) or 70 mesh to 140 mesh (diam. 106 μm to 212 μm). The sphericity and roundness of the proppant particles 110 may also vary based on the desired application.

In some embodiments, the proppant particles 110 may have a rough surface that may increase adhesion of the CO₂-philic coating 120 to the proppant particle 110 and may increase interaction of CO₂ with the CO₂-philic coating 120. The proppant particles 110 may be roughened to increase the surface area of the proppant particle 110 by any suitable physical or chemical method, including, for example, using an appropriate etchant. In some embodiments, the proppant particle 110 may have a surface that provides a desired adherence of the CO₂-philic coating 120 to the proppant particle 110 or may already be sufficiently rough without a need for chemical or physical roughening.

The term “rough” refers to a surface having at least one deviation from the normalized plane of the surface, such as a depression or protrusion. The surface may be uneven and irregular and may have one or more imperfections, such as dimples, stipples, bumps, projections or other surface defects. The rough surface may have an arithmetic average roughness (R_(a)) of greater than or equal to 1 nanometer (nm) (0.001 μm). R_(a) is defined as the arithmetic average of the differences between the local surface heights and the average surface height and can be described by Equation 1, contemplating n measurements:

$\begin{matrix} {R_{a} = {\frac{1}{n}{\sum\limits_{i = 1}^{n}{y_{i}}}}} & {{EQUATION}\mspace{14mu} 1} \end{matrix}$

In Equation 1, each y_(i) is the amount of deviation from the normalized plane of the surface (meaning the depth or height of a depression or protrusion, respectively) of the absolute value of the ith of n measurements. Thus, R_(a) is the arithmetic average of the absolute values of n measurements of deviation y from the normalized plane of the surface. In some embodiments, the surface of the proppant particle 110 may have an R_(a) of greater than or equal to 2 nm (0.002 μm), or greater than or equal to 10 nm (0.01 μm), or greater than or equal to 50 nm (0.05 μm), or greater than or equal to 100 nm (0.1 μm), or greater than or equal to 1 μm.

As previously described, “CO₂-philic” refers to a molecule, ion, polymer, or composition having an affinity or attraction to CO₂. A CO₂-philic material possesses a tendency to mix with, dissolve in, or be wetted by CO₂. In some embodiments, the CO₂-philic material may be attracted to CO₂ molecules through intermolecular forces such as van der Waals forces or through hydrogen bonding. The CO₂-philic coating 120 may be any coating formed from a material or combination of materials that has a chemical or physical affinity to CO₂. The physical affinity of the CO₂-philic coating 120 may include, for example, being capable of absorbing or adsorbing CO₂ molecules.

In some embodiments, the CO₂-philic coating 120 may include or may consist of oxygen-containing molecules such as polysaccharide acetates, polyethylene glycols, ethylene glycol-containing polymers, partially fluorinated oxygen-containing polymers, oxygenated polymers, crosslinked oxygen-containing polystyrenes, polyvinyl acetates, or combinations thereof. For instance, in some embodiments, the CO₂-philic coating 120 may be a polysaccharide acetate, such as cellulose acetate. The CO₂-philic coating 120 in some embodiments may include CO₂-philic materials having silicon-containing groups. Examples of CO₂-philic materials having silicon-containing groups include polysiloxanes, such as those which include monomeric units such as alkylsiloxanes, fluoroalkylsiloxanes, or chloroalkylsiloxanes. Examples of suitable alkylsiloxanes include dimethyl siloxanes and polydimethylsiloxanes. The CO₂-philic coating 120 may comprise halogen compounds (such as fluorine-containing compounds) or compounds having halogenated carbons (for example, fluorocarbons). The CO₂-philic coating 120 may comprise branched polyalkylene oxides or fluorinated polyethers, for example. In some embodiments, the CO₂-philic coating 120 may comprise a fluoropolymer. In some embodiments, the CO₂-philic coating 120 may comprise ethylene glycol-containing polymers made from di(ethylene glycol) monomethyl ether methacrylate or poly(ethylene glycol) methyl ether methacrylate crossklinked with ethylene glycol dimethacrylate or di(ethylene glycol) dimethacrylate (DEGDMA).

In some embodiments, the CO₂-philic coating 120 may be formed from monomeric materials or from oligomeric materials such as, for example, oligomers having 8 or fewer monomer units. The CO₂-philic coating 120 may be an oligomer containing less than or equal to 8 repeating monomers, such as 5 or fewer repeating monomers or 3 or fewer repeating monomers. In some embodiments, the CO₂-philic coating 120 may be a fluoride-containing oligomer. The CO₂-philic coating 120 may be a fluoride-containing oligomer having 8 or fewer repeating monomer units, or 5 or fewer repeating monomer units, or 3 or fewer monomer units. The fluoropolymers may be formed from monomers including, as non-limiting examples, fluoroacrylate monomers such as 2-(N-ethylperfluorooctane-sulfonamido) ethyl acrylate (“EtFOSEA”), 2-(N-ethylperfluorooctane-sulfonamido) ethyl methacrylate (“EtFOSEMA”), 2-(N-methylperfluorooctane-sulfonamido) ethyl acrylate (“MeFOSEA”), 2-(N-methylperfluorooctane-sulfonamido) ethyl methacrylate (“MeFOSEMA”), 1,1′-dihydroperfluorooctyl acrylate (“FOA”), 1,1′-dihydroperfluorooctyl methacrylate (“FOMA”), 1,1′,2,2′-tetrahydroperfluoroalkylacrylates, 1,1′,2,2′-tetrahydroperfluoroalkyl-methacrylates and other fluoromethacrylates; fluorostyrene monomers such as α-fluorostyrene and 2,4,6-trifluoromethylstyrene; fluoroalkylene oxide monomers such as hexafluoropropylene oxide and perfluorocyclohexane oxide; fluoroolefins such as tetrafluoroethylene, vinylidine fluoride, and chlorotrifluoroethylene; and fluorinated alkyl vinyl ether monomers such as perfluoro(propyl vinyl ether) and perfluoro(methyl vinyl ether).

The CO₂-philic coating 120 may comprise a polyether-based polymer including, but not limited to polyethers substituted with at least one side group, which may include one or more groups that interact favorably with or has an affinity for CO₂ (such as a Lewis base group), a poly(ether-carbonate), a poly(ether-carbonate) substituted with at least one side group including a Lewis base, a vinyl polymer substituted with at least one side group including a Lewis base, a poly(ether-ester) or a poly(ether-ester) substituted with at least one side group including a Lewis base. One possible non-limiting example of a Lewis base is an amino functional group.

While embodiments of suitable CO₂-philic materials were described mainly with reference to the CO₂-philic coating 120, it should be understood that the CO₂-philic materials are equally applicable to the swollen CO₂-philic coating 121, which may be in accordance with any of the embodiments of CO₂-philic materials previously described.

In some embodiments, the CO₂-philic coating 120 may have hydrophobic tendencies, such as a lack of attraction to water, repulsion to water, or immiscibility in water. The CO₂-philic coating 120 may not substantially dissolve (does not dissolve more than 10 weight percent (wt. %) or more than 8 wt. %, or more than 5 wt. % or more than 3 wt. %) when contacted with, submerged in, or otherwise exposed to water. In some embodiments, the CO₂-philic coating 120 may not dissociate from the proppant particle 110 when the self-suspending proppant 140 is added to a water-based fluid, such as water or a fluid that includes water. In some embodiments, the CO₂-philic coating 120 of a coated proppant particle 142 or the swollen CO₂-philic coating 121 of a swollen self-suspending proppant 140 does not dissociate from the proppant particle 110 when the self-suspending proppant 140 is contacted, exposed, or placed in a fluid medium having at least 20 wt. % water based on the total weight of the fluid medium. In other embodiments, the CO₂-philic coating 120 of a coated proppant particle 142 or the swollen CO₂-philic coating 121 of a swollen self-suspending proppant 140 does not substantially dissociate (does not dissipate more than 10 wt. %, or more than 8 wt. %, or more than 5 wt. % or more than 3 wt. %) from the proppant particle 110 when the self-suspending proppant 140 is contacted, exposed, or placed in a fluid medium having at least 30 wt. % water, or at least 50 wt. % water, or at least 75 wt. % water. Dissolution of the CO₂-philic coating in a fluid medium may be determined by any suitable analytical technique for detection of solvated coating material that is performed on a fluid medium to which a coated proppant particle 142 has been added and allowed to equilibrate at room temperature for at least 24 hours.

Similarly, the CO₂-philic coating 120 may repel water, may not be attracted to water, or may not be miscible in water. Therefore, in some embodiments, the CO₂-philic coating 120 of a coated proppant particle 142 may not swell when the coated proppant particle 142 is added to a water-based fluid, such as water or a fluid containing water. In some embodiments, the CO₂-philic coating 120 of a coated proppant particle 142 may not swell when the coated proppant particle 142 is contacted with, exposed to, or placed in a fluid medium having at least 20 wt. % water based on the total weight of the fluid medium. In other embodiments, the CO₂-philic coating 120 may not swell when the coated proppant particle 142 is present in a fluid medium having at least 30 wt. % water, or at least 50 wt. % water, or at least 75 wt. % water.

As a non-limiting example, a batch of coated proppant particles 142 having the same CO₂-philic coating 120 is tested in which half of the batch is added to a CO₂-based fluid 130 and the other half of the batch is added to water. The CO₂-philic coating 120 of the coated proppant particles 142 added to the CO₂-base fluid exhibit a swelling (as measured based on the volumetric expansion of the particles before and after addition to the fluid after 30 minutes, as previously described) of at least 2 times, at least 3 times, at least 5 times, at least 50 times, or at least 100 times the amount of the swelling the coated proppants 142 added to water exhibit. In some embodiments, when added to water, the CO₂-philic coating 120 of a coated proppant particle 142 may swell less than or equal to 1/10 the amount the CO₂-philic coating 120 of a coated proppant particle 142 swells when added to a CO₂-based fluid 130. In some embodiments, the CO₂-philic coating 120 of a coated proppant particle 142 when added to water may swell less than or equal to ½, or less than or equal to ⅓, or less than or equal to 1/100, or less than or equal to 1/1,000 the amount the CO₂-philic coating 120 of a coated proppant particle 142 swells when added to a CO₂-based fluid 130.

Referring again to FIG. 1, in one or more embodiments, the proppant particle 110 may be coated with a CO₂-philic coating 120 during a coating step 310 to produce, form, or result in a swollen self-suspending proppant 140. In some embodiments, the CO₂-philic coating 120 may be a surface layer on or bound to the proppant particle 110. Such a surface layer may cover at least a portion of the surface of the proppant particle 110. For example, the CO₂-philic coating 120 may coat, overlay, enclose, envelop, or otherwise surround the proppant particle 110 with the coating. The CO₂-philic coating 120 may cover the entire surface of the proppant particle 110 (as shown) or, alternatively, may only partially surround the proppant particle 110 (not shown), leaving at least a portion of surface of the proppant particle 110 uncoated or otherwise exposed.

The CO₂-philic coating 120 in some embodiments may be lightly crosslinked. As used throughout this disclosure, “lightly crosslinked” refers to partial crosslinking, meaning that at least one crosslinkable site is not crosslinked. For some embodiments described throughout this disclosure, a “lightly crosslinked” CO₂-philic coating 120 may exhibit volumetric swelling of at least 25% at room temperature in the presence of CO₂-based fluid to form the swollen CO₂-philic coating 121. Without intent to be bound by theory, it is believed that greater degrees of crosslinking in the CO₂-philic coating 120 may result in a maximum volumetric swelling of the swollen CO₂-philic coating 121 of less than 25% at room temperature in the presence of CO₂-based fluid. Furthermore, it is believed that in some highly crosslinked polymeric CO₂-philic materials, the rigidity of the polymer segments between crosslink junctions are short and inflexible, such that swelling does not occur when carbon dioxide solvates the polymer segments of such materials. The degree of crosslinking may be controlled by the molar or weight ratio of crosslinkers to monomers used as reactants for forming the CO₂-philic coating 120. A suitable degree of crosslinking may be determined empirically by measuring degrees of swelling in different CO₂-based fluids as a function of temperature and pressure for polymers prepared with varying crosslinker to monomer ratios. Additionally, highly crosslinked coatings may not exhibit an optimal physical structure for constraining or accommodating CO₂ and, therefore, may not exhibit optimal volumetric expansion from the non-swollen state to the swollen state. In contrast, a lightly crosslinked swollen CO₂-philic coating 121 may retain its shape without dissolving in the fluid system, while maintaining a sufficient attraction or bond to the proppant particle 110.

In some embodiments, a lightly crosslinked CO₂-philic coating 120 may constrain CO₂ molecules. In some embodiments, the light crosslinking may create a polymeric network having spaces or voids that accommodate CO₂ molecules. Without intent to be bound by theory, it is believed that the physical structure of the coating may draw in CO₂ as a result of an attraction between the CO₂-philic coating 120 and CO₂ molecules to form the swollen CO₂-philic coating 121. As the CO₂ molecules are drawn into the CO₂-philic coating 120, the CO₂-philic coating 120 may constrain or be solvated with one or more CO₂ molecules to form the swollen CO₂-philic coating 121. In turn, the coated proppant particle 142 may undergo a volumetric expansion from a non-swollen to a swollen state, resulting in a swollen self-suspending proppant 140. Constraint of one or more CO₂ molecules may occur by sorption, for example. Sorption may include physical or chemical adsorption of the CO₂ molecules, physical or chemical absorption of the CO₂ molecules, or any combination of these.

As used throughout this disclosure, “volumetric expansion (E)” refers to the difference of the volume (V₁) of a particular number of coated proppant particles in the swollen state and the volume (V₀) of the same number of coated proppant particles in the non-swollen state, divided by the volume (V₀) of the coated proppant particles in the non-swollen state in accordance with Equation 2:

E=(V ₁ −V ₀)/(V ₀)  EQUATION 2

To express volumetric expansion as a percent, E may be multiplied by 100. The respective volumes of a coated proppant particle 142 (in the non-swollen state) and the volume of a swollen self-suspending proppant 140 (in the swollen state) include the combined volume of both the proppant particle 110 and the CO₂-philic coating 120.

In some embodiments, the coated proppant particle 142 may volumetrically expand from a non-swollen state to a swollen state. The CO₂-philic coating 120 of the coated proppant particle 142 may volumetrically expand when CO₂ is constrained within the physical structure of the coating, forming a swollen CO₂-philic coating 121 resulting in a swollen self-suspending proppant 140. In some embodiments, the swollen self-suspending proppant 140 may exhibit a volumetric expansion of at least 100% from a non-swollen state to a swollen state. In some embodiments, the swollen self-suspending proppant 140 may volumetrically expand at least 10%, at least 15%, at least 20%, at least 25%, at least 50%, at least 200%, or at least 300% from the non-swollen state to the swollen state. Without being bound by theory, such an expansion may increase the buoyancy of the swollen self-suspending proppant 140 and may facilitate or give rise to the suspension of particles in CO₂-based fluids 130, as shown in FIG. 2B.

In some embodiments, at least in part because of this volumetric expansion, the density of the swollen self-suspending proppants 140 may be less than, equal to, or only slightly greater than the density of the CO₂-based fluid 130. In some embodiments, the swollen self-suspending proppant 140 may have a density of less than or equal to 10% of the density of the CO₂-based fluid 130, for example. The density of the swollen self-suspending proppant 140 may be less than or equal to 50%, or less than or equal to 70%, or less than or equal to 85%, or less than or equal to 90% of an unswollen proppant. In some embodiments, one or more viscosifiers may be added to the CO₂-based fluid 130, such that the density of the swollen self-suspending proppant 140 may be less than or equal to 100%, or less than or equal to 125%, or less than or equal to 150%, or less than or equal to 200%, or less than or equal to 250%, or less than or equal to 300% of the density of the CO₂-based fluid 130. The density of the swollen self-suspending proppant 140 may be from 25% to 200% of the density of the CO₂-based fluid 130. In some embodiments, the density of the swollen self-suspending proppant 140 may be from 25% to 125%, or from 50% to 200%, or from 50% to 125%, or from 75% to 150%, or from 75% to 200%, or from 25% to 100%, or from 25% to 110% of the density of the CO₂-based fluid 130. The difference in density may cause the swollen self-suspending proppants 140 to suspend in the CO₂-based fluid 130 without sinking to the bottom or otherwise coalescing in the fluid. The difference in density may reduce or eliminate the need for additional viscosifiers to be added to the CO₂-based fluid 130.

As previously described, a “CO₂-based fluid” refers to a fluid that includes CO₂. In some embodiments, the CO₂-based fluid 130 may comprise at least 30 wt. % CO₂, based on the total weight the CO₂-based fluid 130 before the CO₂-based fluid 130 is contacted with a swollen self-suspending proppant 140. In some embodiments, the CO₂-based fluid 130 may comprise at least 45 wt. % or at least 51 wt. % CO₂. In some embodiments, the CO₂-based fluid 130 may comprise at least 60 wt. %, or at least 75 wt. %, or at least 85 wt. %, or at least 90 wt. % CO₂. In some embodiments, the CO₂-based fluid 130 may comprise liquid CO₂ or gaseous CO₂. In some embodiments, the CO₂-based fluid 130 may comprise supercritical, subcritical, or critical CO₂.

As used throughout this disclosure, “supercritical” refers to a substance at a pressure and a temperature greater than those of the substance's critical point, such that distinct phases do not exist and the substance may exhibit the diffusion of a gas while dissolving materials like a liquid. Similarly, “subcritical” refers to a substance with a pressure and a temperature of less than those of the substance's critical point, such that distinct phases exist. Likewise, “critical” refers to a substance that is at a pressure and temperature equal to those of the substance's critical point. In some embodiments, the CO₂-based fluid 130 may be supercritical CO₂. In some embodiments, the CO₂-based fluid 130 may be liquid CO₂. In some embodiments, the CO₂-based fluid 130 may include substances other than CO₂. The CO₂-based fluid 130 may contain additional fluids or gasses, including but not limited to N₂, dimethyl ether, or hydrocarbons.

Still referring to FIG. 1, further embodiments of the present disclosure are directed to methods for producing self-suspending proppants 140, 142. In some embodiments, the method may include a coating step 310 in which a proppant particle 110 is coated with a monomeric precursor material capable of forming a CO₂-philic material upon polymerization to form a self-suspending proppant. The precursor material may include monomers and, optionally one or more crosslinking agents. In some embodiments, as previously discussed, the degree of crosslinking may be controlled by the molar or weight ratio of crosslinkers to monomers to achieve a coating that is lightly crosslinked.

In some embodiments, the coating step 310 may include contacting the polymerizable precursor material with the proppant particle 110 in a fluidized bed process. In some embodiments, the coating step 310 may include a stationary, bubbling, circulation, or vibratory fluidized bed process. In some embodiments, the coating step 310 may include spraying or saturating the proppant particles 110 with a CO₂-philic polymer or polymer precursor. The coating step 310 may include, in some embodiments, tumbling or agitating the coated proppant to prevent agglomeration or clumping. The coating step 310 may include mixing a CO₂-philic material with another compound such as, for example, a solvent, an initiator, an adhesion promoter, or an additive, to form the CO₂-philic coating 120. In some embodiments, the coating process may be conducted in an emulsion coating technique. In some embodiments, the adhesion promoter may comprise a silane (for example, amino silane) or a silane-containing monomer. In some embodiments, an adhesion promoter may not be necessary to coat the proppant particles 110.

The methods for producing a self-suspending proppant may include polymerizing a polymerizable precursor material that has been coated onto the proppant particle 110 during the coating step 310. In some embodiments, the polymerizable precursor materials may incorporate a polymerization initiator compound. In some embodiments, polymerizing the polymerizable precursor material may include heating the coated proppant to a temperature sufficient to polymerize at least a portion of the precursor material. The heating may include application of heat by any suitable process such as by forced hot air heating, convection, friction, conduction, combustion, exothermic reactions, microwave heating, or infrared radiation, for example. The coated proppant particle may be heated at a polymerization temperature and for a polymerization time sufficient to crosslink at least a portion of the polymerizable precursor material. In some embodiments, the polymerization step may include subjecting the coated proppant to ultraviolet (UV) light or any other polymerization techniques known in the art. The precursor material may be polymerized at conditions sufficient to crosslink at least a portion of the precursor material.

In some embodiments, the methods for producing a self-suspending proppant may further comprise roughening the proppant particles 110 before the coating step 310. The proppant particles 110 may be chemically or physically roughened, as previously described. In some embodiments, roughening the proppant particles 110 may produce rough particle surfaces with an arithmetic average roughness (R_(a)) of greater than or equal to 0.001 μm (1 nm). In some embodiments, the rough particle surfaces may have an R_(a) of greater than or equal to 0.002 μm (2 nm), 0.005 μm (5 nm), or greater than or equal to 0.01 μm (10 nm), or greater than or equal to 0.05 μm (50 nm), or greater than or equal to 0.1 μm (100 nm), or greater than or equal to 0.5 μm, or greater than or equal to 1 μm when measured as previously discussed.

The method for producing a self-suspending proppant may include coating the proppant particle 110 using a two-layer coating or multi-layered coating system. The method in some embodiments may further include coating proppant particles 110 with a top coating. The top coating may be an overlying layer that may be added for additional properties or features. As a non-limiting example, additional coatings may be used in conjunction with, or may comprise a breaker. As used throughout this disclosure, a “breaker” refers to a compound that may break or degrade the polymers of the coating after a fracturing operation to prevent formation damage. In some embodiments, the breaker may be an oxidizer or enzyme breaker. The breaker may be any suitable materials capable of degrading a coating material.

Referring to FIG. 2A, a container 160 of conventional hydraulic fracturing fluid 152 is schematically illustrated to show how uncoated proppant particles 110 react when added to a CO₂-based fluid 130. The conventional hydraulic fracturing fluid 152 includes uncoated proppant particles 110 in a CO₂-based fluid 130. As shown, the uncoated proppant particles 110 sink to the bottom of the container 160, as the conventional hydraulic fracturing fluid 152 is not sufficiently viscous to support the particles without the aid of additional viscosifying means. Settled proppant particles 110 may cause problems in hydraulic fracturing processes. Mitigation of these problems typically requires use of turbulence, or additions of viscosifiers or other ingredients to the conventional CO₂-based hydraulic fracturing fluid 152 to ensure that the proppant particles 110 suspend in the CO₂-based fluid 130. These additional required steps and products may increase the time and costs associated with subterranean treatment.

In contrast, FIG. 2B depicts a container 160 of hydraulic fracturing fluid 150 according to the present embodiments. FIG. 2B schematically illustrates the effect of a coated proppant particle 142 (as shown in FIG. 1) once placed in CO₂-based fluid 130. Once placed in CO₂-based fluid 130, as previously discussed, the CO₂-philic coating 120 (shown in FIG. 1) of the coated proppant particle 142 volumetrically expands to form a swollen self-suspending proppant 140. The hydraulic fracturing fluid 150 of the present disclosure, as shown in FIG. 2B, contains swollen self-suspending proppants 140 including proppant particles 110 coated with a swollen CO₂-philic coating 121. Without being bound by theory, the swollen self-suspending proppants 140 of the present disclosure may overcome the difficulties faced when using conventional hydraulic fracturing fluid 152, as previously discussed, by stabilizing a suspension in hydraulic fluid through the ability of the CO₂-philic coating 120 to volumetrically expand upon contact with CO₂ to result in a swollen CO₂-philic coating 121. The swollen self-suspending proppant 140 may be in accordance with any of the embodiments previously described in which the swollen self-suspending proppant 140 comprises a proppant particle 110 that has been coated with a CO₂-philic coating 120 and contacted with CO₂ to form the swollen CO₂-philic coating 121.

The hydraulic fracturing fluid 150 of FIG. 2B contains swollen self-suspending proppants 140 suspended in the CO₂-based fluid 130. As discussed, in some embodiments, the CO₂-philic coating 120 may at least partially constrain (such as through sorption) CO₂ present in the CO₂-based fluid 130 to produce the swollen CO₂-philic coating 121. The constrained CO₂ molecules may cause the CO₂-philic coating 120, and thus, the swollen self-suspending proppants 140, to volumetrically expand to a swollen state. The swollen self-suspending proppants 140 may become less dense, more buoyant in the CO₂-based fluid, or both, as a result of the constrained CO₂ molecules in the swollen CO₂-philic coating 121. The decreased density or increased buoyancy of the swollen self-suspending proppants 140 may facilitate or enable the particles to self-suspend in the CO₂-based fluid 130.

Embodiments of methods for preparing a hydraulic fracturing fluid 150 may include contacting the coated proppant particle 142 of any of the embodiments previously discussed with a CO₂-based fluid 130 containing CO₂ to form a hydraulic fracturing fluid 150. The hydraulic fracturing fluid 150 may be in accordance with any of the embodiments previously described. Upon contact with the CO₂-based fluid 130 the CO₂-philic coating 120 of the coated proppant particle 142 may swell to a swollen CO₂-philic coating 121, producing a swollen self-suspending proppant 140. The swollen self-suspending proppants 140 may suspend within the hydraulic fracturing fluid 150 without any need for, or with a reduced need for, additives such as viscosifiers. However, the hydraulic fracturing fluid 150 in some embodiments may contain any additives commonly used in the art of fracking, including viscosifiers, for example. The hydraulic fracturing fluid 150 may contain biocides, breakers, buffers, stabilizers, diverting agents, fluid loss additives, friction reducers, iron controllers, surfactants, gel stabilizers, and viscosifiers. In some embodiments, the hydraulic fracturing fluid 150 may not contain any additional viscosifiers. In some embodiments, the hydraulic fracturing fluid 150 may include less than or equal to 30 wt. %, or 20 wt. %, or 15 wt. % or 10 wt. % of any additional viscosifiers. In an embodiment, fracturing fluid having CO₂ and hydrocarbons may contain viscosifiers for hydrocarbons such as alkyl phosphonates, in combination with iron (3+) or aluminum (3+) crosslinkers.

FIG. 3 is a schematic diagram of a subterranean formation being treated using hydraulic fracturing fluid containing proppants to propagate a fracture. As previously discussed, suspended proppant particles 110 within a hydraulic fracturing fluid 150 may aid in treating subterranean fractures, to prop open and keep open the fracture. FIG. 3 shows a subterranean fracture 260 that has been injected with hydraulic fracturing fluid 150 in accordance with the present embodiments containing swollen self-suspending proppants 140, according to the embodiments previously shown and described.

The subterranean fracture 260 of FIG. 3 was generated by contacting a subterranean formation with hydraulic fracturing fluid 150 and using the hydraulic fracturing fluid 150 to propagate and further open the subterranean fracture 260. The hydraulic fracturing fluid 150 in the subterranean fracture 260 of FIG. 3 comprises swollen self-suspending proppants 140 suspended in a CO₂-based fluid 130. In some embodiments, the swollen self-suspending proppants 140, owing in part to their increased buoyancy, may be distributed throughout the CO₂-based fluid 130. The swollen self-suspending proppants 140, as discussed previously, may have a density or buoyancy that prevents the particles from aggregating or otherwise coalescing within the subterranean fracture 260, owing in part to the swollen CO₂-philic coating 121. The hydraulic fracturing fluid 150 may be pumped into the subterranean fracture 260 or may be otherwise contacted with the subterranean formation. In some embodiments the hydraulic fracturing fluid 150 may be pressurized.

Embodiments of methods of treating a subterranean formation may include contacting the subterranean formation with the hydraulic fracturing fluid 150 that includes swollen self-suspending proppants 140 and a CO₂-based fluid 130 in accordance with any of the embodiments previously discussed. Such methods of treating a subterranean formation may include propagating at least one subterranean fracture 260 in the subterranean formation to treat the subterranean formation. In some embodiments, the subterranean formation may be a rock or shale formation. In some embodiments, contacting of the subterranean formation may include drilling into the subterranean formation and subsequently injecting the hydraulic fracturing fluid 150 into at least one subterranean fracture 260 in the subterranean formation. In some embodiments, the hydraulic fracturing fluid 150 may be pressurized before being injected into the subterranean fracture 260 in the subterranean formation.

Though embodiments of the present disclosure have been discussed in the context of hydraulic fracturing processes, embodiments of the present disclosure may also be used in other industries. For example, in some embodiments, the swollen self-suspending proppants 140 and hydraulic fracturing fluid 150 of the present disclosure may be used to stimulate groundwater wells, to precondition or induce rock cave-ins for mining operations, to dispose of waste by injecting it deeply into rock, to measure stresses in the Earth's crust, to generate electricity in enhanced geothermal systems, or to increase injection rates for the geologic sequestration of CO₂.

It should be apparent to those skilled in the art that various modifications and variations may be made to the embodiments described within without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover the modifications and variations of the various embodiments described within provided such modification and variations come within the scope of the appended claims and their equivalents.

Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed within should not be taken to imply that these details relate to elements that are essential components of the various embodiments described within, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Further, it should be apparent that modifications and variations are possible without departing from the scope of the present disclosure, including, but not limited to, embodiments defined in the appended claims. More specifically, although some aspects of the present disclosure are identified as particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects. 

What is claimed is:
 1. A hydraulic fracturing fluid comprising: a fluid comprising CO₂, and self-suspending proppants suspended in the fluid, the self-suspending proppants comprising proppant particles coated with a CO₂-philic coating in which the CO₂-philic coating is lightly crosslinked and has a physical structure that constrains CO₂ molecules.
 2. The hydraulic fracturing fluid of claim 1, in which the fluid further comprises N₂.
 3. The hydraulic fracturing fluid of claim 1, in which the physical structure of the CO₂-philic coating comprises constrained CO₂ molecules that volumetrically expand the self-suspending proppants to a swollen state.
 4. The hydraulic fracturing fluid of claim 3, in which the physical structure of the CO₂-philic coating is volumetrically expanded by at least 100% from a non-swollen state to the swollen state.
 5. The hydraulic fracturing fluid claim 1, in which the self-suspending proppants in a swollen state have a density of less than or equal to 200% of the density of the fluid.
 6. The hydraulic fracturing fluid of claim 1, in which the proppant particles have rough surfaces with an arithmetic average roughness (R_(a)) of greater than or equal to 1 nm.
 7. The hydraulic fracturing fluid of claim 1, in which the CO₂-philic coating comprises a CO₂-philic material chosen from polysaccharide acetates, polyethylene glycols, partially fluorinated oxygen-containing polymers, oxygenated polymers, crosslinked oxygen-containing polystyrenes, polyvinyl acetates, and combinations of any of these.
 8. The hydraulic fracturing fluid of claim 1, in which the CO₂-philic coating does not dissociate from the proppant particles when the self-suspending proppants are added to a water-based fluid. 